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Topic Review (Newest First)

02-07-2012 11:02 AM

1971BB427

The 4-7 swap is what some racers in the 60's changed to in an effort to smooth up the engine and get a litle higher rpm's. It involves having a cam ground to this configuration, or buying one. (they are available)
The 4-7 swap is in my opinion a lot of work and money for very small gains, and not worth the time and money.

02-06-2012 09:13 PM

blight

what is your opinion on 4-7 swaps etc?

02-06-2012 09:08 PM

oldbogie

Quote:

Originally Posted by PatM

If you do it, please do it online so that folks NOT in the great NorthWest can share in the fun.

Pat

More detail than I know just yet.

Using your response to expose the guys to the Buick Nailhead firing order. This is the same cylinder counting as the Chev but a way different firing order of 1-2-7-8-4-5-6-3.

OK I'm locking up and going home.

Bogie

02-06-2012 02:07 PM

PatM

If you do it, please do it online so that folks NOT in the great NorthWest can share in the fun.

Pat

02-06-2012 01:36 PM

oldbogie

Quote:

Originally Posted by blight

I happen to be doing some reading recently on (ferrari, datsun L6 motors etc) v12s and L6s and angle of crankshafts etc. I came across some info on why v12s and L6 motors are "naturally balanced". Apparently aside from crankshaft angles uses of flat plane crankshafts, one of the biggest factors was the firing order of the engines and how that then affects the throw of each cylinder balance the other piston throws (in basic terms).

So - that being said, theoretically, what firing order would a sbc need to have to be balance like these motors?

Has nothing to do with firing order, the issue is of secondary balance which is having a cylinder's parts in motion off setting the actions of another. In this case an inline 6 or a 60, 120,180 degree V12 have such a situation because of block and crankshaft configuration.

A 90 degree V8 always has a little secondary vib, there's no way around it by juggling firing orders or anything else short of a counter balance shaft.

The I6, V12 and V8 have primary balance which is the rotational masses in balance. The secondary balance the is motion of the reciprocating parts. Their motions are not linear with crankshaft degrees. Each piston with the rod small ends and some amount of shank have different rates of acceleration as the crankshaft turns and the reciprocating parts move up and down rather than round and round as the crank and big end of the rod do. For secondary balance there needs to be a combination of two cylinders worth of rods and pistons that are doing exactly the same acceleration but in reverse directions of motion to each other.

By "acceleration" I'm talking the different speeds the piston and upper rod travel at even when the engine itself is held to a constant RPM. This has nothing to do with speeding or slowing the crankshaft.

It's all in high school trigonometry class. Think how much more fun it would be with engines for examples instead of dry memorization of trig functions and the doing of inane problems. The SAE keeps bugging me to become a math mentor, maybe I should. That would be one bizarre math class.

When comparing the firing orders, be sure you take the way the cylinders are numbered into consideration.

As an example the Ford "1-5-4-2-6-3-7-8" is (when looked at using Chevy cylinder numbering) is the same as Chevy: 1-8-4-3-6-5-7-2.

The Northstar, IIRC, has #1 on the left side looking at the "front" of the engine, driver side transversly.

02-06-2012 10:37 AM

cobalt327

The way the SBC was originally designed has served that engine well for a long time and that's what all the aftermarket and production cranks are designed around. Since there's basically only the 4/7 swap to change to (unless you were starting w/a blank sheet or were going to use a 180 degree crank or split rod journals like even/semi-even fire V6 engines use), I'd leave it be.

Regardless of which of these firing orders are used, there will be two cylinders on each bank firing consecutively and once the two cylinders will be adjacent cylinders that fire consecutively.

I hope Old Bogie or Techinspector1 notices this thread and fills in the many blanks/details.

02-06-2012 12:01 AM

blight

Why would this be moved to general rodding tech? This is particular about engines- where this originally was...

02-05-2012 11:44 PM

blight

Sbc firing order changes?

I happen to be doing some reading recently on (ferrari, datsun L6 motors etc) v12s and L6s and angle of crankshafts etc. I came across some info on why v12s and L6 motors are "naturally balanced". Apparently aside from crankshaft angles uses of flat plane crankshafts, one of the biggest factors was the firing order of the engines and how that then affects the throw of each cylinder balance the other piston throws (in basic terms).

So - that being said, theoretically, what firing order would a sbc need to have to be balance like these motors?